(1) Field of the Invention
The present invention relates to a method and to a device for assisting the piloting of an aircraft, and also to an aircraft, in particular a rotorcraft provided with a main lift and propulsion rotor together with a tail rotor for providing yaw control.
(2) Description of Related Art
Rotorcraft are generally provided with a power plant comprising at least one engine, such as an engine of the free-turbine turboshaft type. Power is then taken from a low-pressure stage of each free turbine that rotates substantially in the range 20,000 revolutions per minute (rpm) to 50,000 rpm. Consequently, the power plant includes a speed-reducing gearbox for connecting the free turbine to the main propulsion and lift rotor, since the speed of rotation of said rotor lies substantially in the range 200 rpm to 400 rpm: this is referred to as the main gearbox.
Temperature limits of a turbine engine and torque limits of a main gearbox are used to define an operating envelope for the turbine engine covering two normal utilization ratings for a turbine engine on a single-engined or twin-engined rotorcraft:
takeoff rating corresponding to a torque level for the main gearbox and to a temperature level for the engine that are acceptable during a limited period of time without significant degradation, with this takeoff rating being defined by a maximum takeoff power (TOP) and a length of time during which this maximum takeoff power TOP may be used, generally of the order of five minutes;
a maximum continuous rating, this maximum continuous rating being defined by a maximum continuous power (MCP) that corresponds to about 90% of the maximum takeoff powered TOP and by a duration of use for this maximum continuous power MCP that is generally unlimited;
an extended power rating, this extended power rating being defined by an extended power that is substantially equivalent to or even equal to the maximum takeoff power TOP and by a duration for utilizing this extended power that is of the order of thirty minutes; and
a transient rating that is determined by regulation of the power plant being defined by a maximum transient power (MTP).
In a twin-engined rotorcraft, the operating envelope also includes supercontingency ratings that are used only when one of the two turbine engines has failed:
the first emergency rating, this emergency rating being defined by a supercontingency power (PSU), sometimes referred to as the “one-engine-inoperative 30-seconds” (OEI 30″) rating, which is often equal to about 112% to 120% of the maximum takeoff power, with a maximum duration for which this supercontingency power can be used that is of the order of thirty consecutive seconds, the supercontingency power conventionally being usable three times during a flight;
a second emergency rating, this second emergency rating being defined by a maximum supercontingency power (PMU) sometimes known as the “one-engine-inoperative two minutes” (OEI 2′) rating, which is equal to about 105% to 110% of the maximum takeoff power, with this power rating being usable for about two consecutive minutes at most; and
a third emergency rating, this third emergency rating being defined by an intermediate supercontingency power (PIU), sometimes referred to as the “one-engine-inoperative continuous” (OEI cont) rating, which is substantially equal to the maximum takeoff power and corresponds to an unlimited duration of utilization of this intermediate supercontingency power for the remainder of the flight after one engine has failed.
By calculation or by testing, the engine manufacturer draws up available power curves for a turbine engine as a function of altitude and of external temperature, with this being done for each of the above-defined ratings.
Furthermore, the engine manufacturer determines limitations for each turbine engine that make it possible to obtain both the powers MCP, TOP, MTP, OEI 30″, OEI 2′, OEI cont corresponding to each of the above-specified ratings, and also an acceptable lifetime. These limits are generally monitored by means of three monitoring parameters of the engine: the speed of rotation of the turbine engine gas generator, the engine torque, and the temperature of the gas at the inlet of the low-pressure free turbine of the turbine engine, these quantities being respectively labeled Ng, Cm, and T45 by the person skilled in the art. If the turbine engine includes a high-pressure turbine stage, it is also possible to use the temperature of the gas at the inlet of the high-pressure turbine (written TET), with this gas temperature at the inlet to the high-pressure turbine being difficult to measure and consequently being calculated on the basis of the gas temperature at the inlet to the free turbine, i.e. T45.
Thus, for each rating of the operating envelope of the engine, the manufacturer draws up limits for each monitoring parameter, where these limits may vary as a function of external conditions, i.e. external pressure P0 and external temperature T0 as present outside the aircraft.
For example, for a single-engined aircraft, the engine manufacturer may determine:
first temperature limits T4limTOP, T4limMCP, and T4limMTP corresponding to the gas temperature at the inlet to the low-pressure free turbine of the engine respectively when the engine is developing maximum takeoff power, maximum continuous power, and maximum transient power, with these first limits varying as a function of external conditions;
second limits NglimTOP, NglimMCP, and NglimMTP corresponding to the speed of rotation of the engine gas generator when the engine is developing respectively maximum takeoff power, maximum continuous power, and maximum transient power, these second limits varying as a function of external conditions; and
third limits TQlimTOP, TQlimMCP, and TQlimMTP corresponding to the torque exerted on the outlet shaft of the engine while the engine is developing respective maximum takeoff power, maximum continuous power, and maximum transient power, these third limits varying as a function of external conditions.
It should be observed that the third limits may be measured by analogy on the basis of the torque exerted by the main gearbox, at the inlet of the main gearbox and/or at its mast for driving the main rotor, for example.
These various limits are established by the engine manufacturer and the aircraft builder, in the form of tables, a database, or equations, for example.
The pilot must therefore control the aircraft while taking into consideration the appropriate limits in order to comply with the recommendations of the engine manufacturer so as to protect the moving assemblies of the helicopter.
In order to control these limits, various piloting assistance devices are known.
Document FR 2 749 545 discloses a piloting indicator that presents information relating to that one of the monitoring parameters of the turbine engine that is the closest to its own limit value. The information relating to the limitations that need to be complied with are thus brought together on a single display, making it possible firstly to provide a summary thereof, presenting only the result of the summary so as to simplify the task of the pilot, and secondly to save space on the instrument panel. This provides a “limiting parameter” from amongst said monitoring parameters of the turbine engine, where the limiting parameter is that one of the parameters that presents a current value that is the closest to the limit value for said parameter. That is why such an indicator is referred to below as a “first limitation indicator”, or (FLI).
For example, the various monitoring parameters are converted to a comparable scale in terms of engine torque after being compared with their respective limits, which limits may be variable in order to take account of external pressure and external temperature, or indeed of levels of power being taken off from the engine.
In addition, variants of this FLI serve to display the value of the limiting parameter in terms of collective pitch margin for the blades of the main rotor of the rotorcraft.
For example document FR 2 756 256 suggests presenting on a display screen the available power margin for the engine before it reaches a limit on a scale that is graduated in equivalent collective pitch for the blades of the main rotor, which scale travels past an index that is representative of the actual collective pitch of said blades. For example, the index may be in register with a first graduation, and the limit of the limiting parameter at a given power may be in register with a second graduation that is greater than the first graduation. The pilot then knows the collective pitch margin that is available before reaching said given power.
Document EP 0 811 183 describes an alternative piloting assistance device.
In a helicopter, it should be understood that the power developed by the power plant is indeed consumed by the main lift and propulsion rotor, but is also consumed by other members, e.g. the tail rotor.
Document FR 2 749 545 suggests taking into consideration the power that is being taken off while establishing the values of the limits. Although that can be advantageous, it will be understood that the pilot might be disturbed on observing a change in said limits, even when the pilot is not acting on the control lever for controlling the collective pitch of the blades of the main rotor.
Documents EP 2 258 615 and US 2005/278084 are also known.